Method for producing oxide superconducting wire

ABSTRACT

A method of manufacturing an oxide superconducting wire, through which dimensional precision of width or thickness of the wire can be improved and an oxide superconducting wire with high superconducting performance can be obtained, is provided. The method of manufacturing the oxide superconducting wire includes preparing a composite by covering with metal powder containing an oxide superconductor or raw material for an oxide superconductor, and rolling the composite using a lubricant having kinematic viscosity of 20×10 −6  mm 2 /s or smaller.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an oxidesuperconducting wire, and specifically, to a method of manufacturing anoxide superconducting wire including a step of rolling a compositeprepared by covering with metal powder containing an oxidesuperconductor or raw material for an oxide superconductor.

BACKGROUND ART

Conventionally, in general, when manufacturing a superconducting wireconsisting of a composite in which an oxide superconductor is coveredwith metal, firstly oxide powder as a raw material for superconductor isfilled in a metal pipe of silver or the like to fabricate a composite ofmonofilamentary structure (a monofilamentary composite). Thereafter, aplurality of the monofilamentary composites are bundled and insertedinto another metal pipe to fabricate a composite of multifilamentarystructure (a multifilamentary composite). On the multifilamentarycomposite, workings such as drawing or rolling are performed to obtain atape-shaped wire. Through a thermal treatment of such tape-shaped wire,a wire with superconduction can be obtained.

In the superconducting wire thus manufactured, properties of interest inpractical use are superconduction such as critical temperature orcritical current value. In addition, improvement of dimensionalprecision of the wire is nevertheless important.

The above mentioned superconducting wire are practically used asimplemented in a magnet or a cable. As a magnet, a coil form with atape-shaped oxide superconducting wire being rolled as a pancake, aswell as a stacked structure of such coils are employed to utilize themagnetic field generated at the center when a current is applied to thesuperconducting wire. As a cable, a structure in which a tape-shapedoxide superconducting wire is wound around the outside of a cylindricaltube is employed. If a tape-shaped superconducting wire is not uniformin width or thickness, then in the magnet form, aligned winding ishindered and thus the shape of the coil is degraded, and additionally,in the cable form, the superconducting wires are overlaid with eachother and thus superconducting properties are degraded by the resultingstress. Therefore, in order to obtain desired superconducting propertieswhen forming the superconducting wire into a magnet or a cable, it isimportant to improve dimensional precision in width and thickness of thetape-shaped superconducting wire.

One manufacturing method for solving above mentioned problems isproposed in Japanese Patent No. 2,951,423 (Japanese Patent Laying-OpenNo. 4-269407) (Prior Invention 1). In Prior Invention 1, a method isemployed in which a composite, obtained by filling a ceramic materialfor a superconductor in a metal pipe, is compression molded using apress-type pressure jig. By utilizing the pressure jig to constrain thewidth of the composite while working, the variation in width of thesuperconducting wire eventually obtained is eliminated.

Another manufacturing method for solving above mentioned problems isproposed in International Publication WO97/48123 (Prior Invention 2). InPrior Invention 2, it is proposed to use various lubricant for rollworking a composite with small coefficient of friction of 0.2 or smallerto improve dimensional precision of a superconducting wire.

With the manufacturing method of Prior Invention 1, if width dimensionof the composite is not precise before being compression formed by thepressure jig having grooves for constraining the width thereof, thendesired shape of superconducting wire can not be obtained at the laststage. For example, if width dimension of the composite before passingthrough the pressure jig is larger than, or close to, that of thegrooves of the pressure jig, then edges in width direction of thecomposite (width-direction edges) will be deformed after beingcompression molded by the pressure jig. This may result in formation ofprotrusions on width-direction edges of the superconducting wireeventually obtained, or generation of breaks such as cracks in oxidesuperconducting filaments within the superconducting wire. In some partsof a composite where the width dimension before passing through thepressure jig is smaller than that of the grooves of the pressure jig,certain portions on width-direction edges of the composite do not abuton the side of the grooves of the pressure jig while being worked withit. Further, in this method, the composite is intermittently compressionformed, resulting in joints existing between worked parts. There is aproblem that at such joints, performance of the superconducting partwithin the wire is degraded.

In the manufacturing method of Prior Invention 2, use of variouslubricants to reduce coefficient of friction is proposed. However, therehas been problems that, depending on the type of the lubricant, rollingthe composite with the lubricant may result in cracks at width-directionedges (edge cracking) of the superconducting wire eventually obtained,or may degrade precision of the dimension.

The above mentioned problems attribute to the fact that the oxidesuperconducting wire is not formed with a single material, but with acomposite of an oxide and a metal. For example, a composite ofmultifilamentary structure contains, in silver of about 100 to 150 Hv inhardness, 60 to 80 filaments formed with aggregated oxide powder havingparticle diameter of about 10 μm. The hardness of such filaments vary ina range of 50 to 100 Hv before being rolled. Because of the variety ofhardness among the filaments and the difference of hardness between thefilaments and silver, dimensional precision is hard to be improved byroll working. Even if dimensional precision is improved, there stillremain problems such as generation of breaks in the filaments within thewire.

Therefore, it is an object of the present invention to provide a methodof manufacturing an oxide superconducting wire, in which dimensionalprecision of width or thickness of the wire can be improved and asuperconducting wire with high superconducting performance can beobtained.

DISCLOSURE OF THE INVENTION

A method of manufacturing an oxide superconducting wire according to thepresent invention includes the step of preparing a composite formed ofpowder containing an oxide superconductor or raw material for an oxidesuperconductor covered with metal, and the step of rolling the compositeusing a lubricant having kinematic viscosity of at most 20×10⁻⁶ mm²/s.

In a manufacturing method of the present invention, by rolling thecomposite using a lubricant having low kinematic viscosity of at most20×10⁻⁶ mm²/s or smaller, formation of cracks can be prevented in theedges of wires eventually obtained, thus dimensional precision of thewire can be improved.

In a manufacturing method of the present invention, kinematic viscosityis preferably at most 10×10⁻⁶ mm²/s.

Further, in a manufacturing method of the present invention, thelubricant preferably has a boiling point of at most 200° C. In thepresent case, by utilizing highly volatile lubricant, the lubricant isvolatilized by a processing heat generated upon roll working, thenlatent heat thereupon cools the pressure rolls. Thus, deformation of thepressure rolls can be prevented, and the rolling precision, and hencedimensional precision of the wire eventually obtained can further beimproved.

Further, in a manufacturing method of the present invention, the step ofrolling preferably includes rolling the composite with rolling reductionof at most 40%. In the present case, by gradually rolling the compositewith small rolling reduction, dimensional precision of the wireeventually obtained can further be improved, especially the variation indimension of the wire can be reduced.

In a manufacturing method of the present invention, the step of rollingpreferably includes rolling the composite using a pressure rolls havinggrooves for constraining width of the composite. In the present case,dimensional precision of the wire eventually obtained, as well as thesuperconducting performance thereof, for example critical current value,can be improved.

Further, in a manufacturing method of the present invention, the step ofperforming roll working preferably includes rolling the composite with afour-directional pressure rolls. In this case, the composite is rolledwhile being constrained by upper, lower, right and left rolls, thus therolling precision, and hence dimensional precision of the wireeventually obtained can further be improved.

In a manufacturing method of the present invention, the step ofpreparing a composite preferably includes filling into a metal pipepowder containing an oxide superconductor or raw material for an oxidesuperconductor.

Further, in a manufacturing method of the present invention, the step ofpreparing the composite preferably includes preparing a monofilamentarycomposite by filling into a first metal pipe powder containing an oxidesuperconductor or raw material for oxide superconductor, and preparing amultifilamentary composite by filling a plurality of monofilamentarycomposites into a second metal pipe. In the present case, the step ofpreparing composite further preferably includes drawing themonofilamentary composite, and the step of preparing themultifilamentary composite includes filling a plurality of drawnmonofilamentary composite in the second metal pipe. In the present case,the step of preparing the composite further preferably includes drawingthe multifilamentary composite.

In a manufacturing method of the present invention, a silver pipe or asilver alloy pipe is preferably employed as the metal pipe.

Further, in a manufacturing method of the present invention, an oxidesuperconductor is preferably a bithmuth-based oxide superconductor. Thebithmuth-based oxide superconductor includes bithmuth, lead, strontium,calcium, and copper having a (bithmuth andlead):strontium:calcium:copper composition ratio approximately expressedas 2:2:2:3.

Further, a method of manufacturing an oxide superconducting wire of thepresent invention preferably further includes a step of thermallytreating the rolled composite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph showing a width-direction edge of an oxidesuperconducting wire manufactured according to the present invention;

FIG. 2 is a micrograph showing a width-direction edge of an oxidesuperconducting wire with edge cracking as a comparative example to thepresent invention;

FIG. 3 is a micrograph showing a cross section of an oxidesuperconducting wire with protrusions on edges of an oxidesuperconducting wire as a comparative example to the present invention;

FIG. 4 is a micrograph showing a cross section of an oxidesuperconducting wire of which superconducting filaments are broken as acomparative example to the present invention;

FIG. 5 is a micrograph showing a longitudinal cross section of an oxidesuperconducting wire of which superconducting filaments are broken as acomparative example to the present invention;

FIG. 6 is a micrograph showing a longitudinal cross section of an oxidesuperconducting wire manufactured according to the present invention;

FIG. 7 illustrates as one embodiment outer profile of a cross sectionalong axial direction of a pair of pressure rolls having grooves used ina manufacturing method of the present invention;

FIG. 8 is a micrograph showing a cross section of an oxidesuperconducting wire obtained by roll working with flat rolls in anembodiment of the present invention;

FIG. 9 is a micrograph showing a cross section of an oxidesuperconducting wire obtained by roll working with rolls having groovesin an embodiment of the present invention;

FIG. 10 is a graph showing changes in width against length of an oxidesuperconducting wire rolled using a lubricant of low volatility as anembodiment of the present invention;

FIG. 11 is a graph showing changes in thickness against length of anoxide superconducting wire rolled using a lubricant of low volatility asan embodiment of the present invention;

FIG. 12 is a graph showing changes in temperature of roll surfaceagainst length of an oxide superconducting wire rolled using a lubricantof low volatility as an embodiment of the present invention;

FIG. 13 is a graph showing changes in width against length of an oxidesuperconducting wire rolled using a lubricant of high volatility as anembodiment of the present invention;

FIG. 14 is a graph showing changes in thickness against length of anoxide superconducting wire rolled using a lubricant of high volatilityas an embodiment of the present invention; and

FIG. 15 is a graph showing changes in temperature of roll surfaceagainst length of an oxide superconducting wire rolled using a lubricantof high volatility as an embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

In one embodiment of the present invention, a step of rolling acomposite using a lubricant having kinematic viscosity of at most20×10⁻⁶ mm²/s is employed. A method using a lubricant is effective forimproving dimensional precision of an oxide superconducting wireeventually obtained, however, breaks called “edge cracking” may begenerated at width-direction edges of the oxide superconducting wireeventually obtained, especially when a lubricant with high viscosity isused, due to effects of hardness or elongation of metal coverings andoxide filaments constituting the composite. The generation of suchbreaks not only degrades superconducting property of the superconductingwire, but eventually deprives it of the function as a superconductingwire. Thus, the lubricant used in the step of rolling must be low inviscosity. In order to prevent the generation of edge cracking in theedges of the oxide superconducting wire eventually obtained, a lubricantwith a kinematic viscosity of at most 20×10⁻⁶ mm²/s must be selected.

Further, even if no edge cracking is generated at the edges of thesuperconducting wire, breaks of small width of about a few nm to a fewμm might be generated at the edges of the superconducting wire. Suchbreaks are generated not only when the metal coating constituting thecomposite is silver, but also when it is a silver alloy. Thesuperconducting wire with such breaks does not exhibit problems withdimensional precision. On the other hand, once the length of thecomposite as a subject of rolling exceeds 100 m, a phenomenon can beobserved in which silver as a metal coating breaks into small dustswhich in turn melt and adhere onto the surface of pressure rolls. Suchmelting phenomenon of silver onto the surface of the pressure rollsdegrades precision in rolling, and hence, dimensional precision of theoxide superconducting wire eventually obtained. In order to prevent suchphenomenon, kinematic viscosity of a lubricant used in the step ofrolling must be at most 10×10⁻⁶ mm²/s.

Rolling of the composite generates heat. This processing heat alsocontributes to degradation of rolling precision. Since the processingheat gradually raises the temperature of the surface of the pressurerolls, they will be deformed. As such, gaps between the pressure rollsvary, and hence, the rolling precision varies.

In order to suppress the processing heat, generally known methods are tocool the pressure rolls with air or water, or to use hot rolls. Whencooling pressure rolls with air or water, the entire pressure rolls arecooled, making it difficult to control subtle fluctuation in temperatureof the surface of the pressure rolls. For example, when variation ofwidth dimension of the superconducting wire eventually obtained isintended to be limited by 300 μm, cooling of pressure rolls with air orwater can hardly suppress locally increasing temperature of the pressurerolls working on the composite. Further, when using hot rolls, sincesilver or silver alloy is used in the metal covering of the composite tobe rolled, a problem arises that the metal covering is softened as thetemperature of the pressure roll increases. The softened silver orsilver alloy covering induces variation in the strength of the metalcovering, and thus improvement of the rolling precision is hard to beachieved.

Therefore, in one example of manufacturing method of the presentinvention, a lubricant which has kinematic viscosity in the abovementioned range, and which is highly volatile is used. Especially, it ispreferable to use a lubricant having boiling point of at most 200° C. asthe highly volatile lubricant. When a highly volatile lubricant is used,the lubricant is volatilized by processing heat generated upon rollworking, then latent heat thereupon cools the pressure rolls. Thus,locally increasing temperature of the pressure roll can be suppressedand thus gaps between pressure rolls are prevented from variation, andthus the rolling precision is improved. As a result, variation indimension of width or thickness of the superconducting wire eventuallyobtained can further be reduced.

In a manufacturing method of the present invention, the rollingprecision can be improved by using prescribed lubricant as above. Tofurther improve the rolling precision, rolling reduction for rolling thecomposite into a tape-shaped superconducting wire must be reduced, wherethe rolling reduction is expressed as:

{(thickness before rolling)−(thickness after rolling)}×100/(thicknessbefore rolling)[%].

For example, even when rolling reduction of 80% is required forprocessing a round shaped composite into a final tape-shaped wire, it ispreferable to roll the composite stepwise with rolling reduction of atmost 40%. By successively rolling the composite stepwise with rollingreduction of at most 40%, dimensional precision of the superconductingwire eventually obtained can be improved.

When performing roll working stepwise as above, it is preferable toprovide grooves to the pressure rolls for constraining width of thecomposite in order to further improve dimensional precision of thesuperconducting wire. In this case, slight variation may be generated inthe dimension of width when rolling, thus a narrow gap must be providedbetween the upper and lower pressure rolls. Especially, rolling withpressure rolls with grooves improves distribution of superconductingfilaments in the cross section of the superconducting wire eventuallyobtained. Additionally in this case, one problem to be solved at thetime of rolling is to improve density of the oxide superconductingfilaments. This is required to improve the performance ofsuperconducting portion of the wire. To meet the requirement, even whenrolling is done stepwise using the grooved pressure roll, it ispreferable to roll the composite using flat rolls as the pressure rollin the last step.

In one manufacturing method of the present invention, the rollingprecision can further be improved by using a four-directional pressureroll at the last stage of roll working, for rolling the compositeconstraining both of its thickness and width. It should be noted thatthe variation of dimensional precision in width of the composite beforebeing introduced in the four-directional pressure roll at the last stepshould be at most 300 μm. If the composite which varies in width morethan 300 μm is introduced in the four-directional pressure roll,protrusions will be generated in the width-direction edges of thesuperconducting wire eventually obtained and thus dimensional precisionis degraded, and additionally breaks will be generated in thesuperconducting filaments within the wire. As a result, thesuperconducting performance of the wire eventually obtained will bedegraded. As such, working with the four-directional pressure roll forincreasing density of the superconducting filaments is not preferable.Additionally, since the purpose of utilizing four-directional pressurerolls is not in densification of superconducting wire, the upper, lower,right and left rolls constituting the four-directional pressure rollsused herein are all preferably non-driving rolls.

(Embodiment)

FIRST EXAMPLE

The respective powders of oxide bithmuth (Bi₂O₃), lead monoxide (PbO),strontium carbonate (SrCO₃), calcium carbonate (CaCO₃), copper oxide(CuO) were mixed together to prepare powder having a Bi:Pb:Sr:Ca:Cucomposition ratio of 1.82:0.33:1.92:2.01:3.02. The obtained powder wasthermally treated at 750° C. for 10 hours, and then 800° C. for 8 hoursto produce a sintered compact. The obtained sintered compact was crushedto powder using an automatic mortar. Next, the powder was thermallytreated at 850° C. for 4 hours and then crushed again using theautomatic mortar. The powder was further thermally treated at 800° C.for 2 hours, and then filled in a silver pipe having an outer diameterof 36 mm and an inner diameter of 30 mm. The silver pipe with the powderfilled therein as a composite was drawn. Thus obtained 61 wires werebundled together, and then inserted into a silver pipe having an outerdiameter of 36 mm and an inner diameter of 31 mm, which in turn wasfurther drawn to a wire having a diameter of 1.5 mm. The wire was rolledto a target tape shape having a thickness of 0.26 mm and a width of 3.7mm, and thus a wire as a composite with a length of 1000 m wasmanufactured.

In the above mentioned rolling step, lubricants with different viscositywere used. Eight types of lubricants, of which kinematic viscositieswere respectively 150×10⁻⁶ mm²/s, 115×10⁻⁶ mm²/s, 45×10⁻⁶ mm²/s, 30×10⁻⁶mm²/s, 20×10⁻⁶ mm²/s, 10×10⁻⁶ mm²/s, 5×10⁻⁶ mm²/s, 3×10⁻⁶ mm²/s, wereprepared and each were used in rolling.

As a result, in the wire as the composite after being rolled withlubricants with kinematic viscosities of 150×10⁻⁶ mm²/s and 115×10⁻⁶mm²/s, numerous breaks in width-direction edges (edge cracking) weregenerated. In the wire as the composite after being rolled withlubricants with kinematic viscosities of 45×10⁻⁶ mm²/s and 30×10⁻⁶mm²/s, breaks (edge cracking) were generated in some part ofwidth-direction edges. Further, rolling with a lubricant with kinematicviscosity of 20×10⁻⁶ mm²/s or lower, no breaks were observed in theappearance of the wire as the composite after being rolled.

FIG. 1 is a micrograph (magnification of 64) showing magnifiedwidth-direction edges of the wire with no breaks. FIG. 2 is a micrograph(magnification of 64) showing magnified width-direction edges of thewire with breaks.

SECOND EXAMPLE

In the above mentioned rolling step of the first example, mineral oil ofwhich kinetic velocity is in the range of 20×10⁻⁶ mm²/s to 3×10⁻⁶ mm²/swas used as a lubricant. The precision of the width and the thickness ofthe wire thus obtained were measured. The results are shown in Table 1.

TABLE 1 kinematic viscosity width (mm) thickness (mm) (× 10⁻⁶ standardmaximum minimum standard maximum minimum mm²/s average deviation valuevalue average deviation value value 20 3.70 0.08 3.94 3.44 0.263 0.0100.287 0.234 10 3.69 0.04 3.80 3.55 0.259 0.006 0.270 0.250 5 3.70 0.043.80 3.52 0.259 0.004 0.267 0.251 3 3.68 0.03 3.81 3.64 0.259 0.0040.265 0.250

As seen from the results of Table 1, it can be appreciated that usinglubricants having kinematic viscosity of 20×10⁻⁶ mm²/s or lower,dimensional precision of width and thickness of the wire can further beimproved.

THIRD EXAMPLE

Each of the wires obtained by the second example were rolled with theirwidth constrained by a four-directional pressure roll in whichnon-driving four rolls (upper, lower, right, and left rolls) werecombined. The rolling reduction was 3%. The states of the wires afterrolling were evaluated.

As a result, it was found that protrusions were formed in thewidth-direction edges only on the wire rolled using a lubricant withkinematic viscosity of 20×10⁻⁶ mm²/s. The cross section of the wire waspolished and the oxide superconducting filaments were observed, andbreaks were generated therein.

FIG. 3 is a micrograph (magnification of 36) showing magnifiedwidth-direction edges of a tape-shaped oxide superconducting wire withprotrusions. In FIG. 3, it can be appreciated that the width-directionedges are bulging, indicating formation of protrusions. The white partis a silver-covered portion, and the black part is the oxidesuperconducting filaments. In the cross section of the wire shown inFIG. 3, it can be seen that the oxide superconducting filaments aregreatly deformed in width-direction edges.

From the above results, it can be appreciated that protrusions areformed in the width-direction edge, or breaks are generated in the oxidesuperconducting filaments when a wire, rolled with a lubricant havingkinematic viscosity of 20×10⁻⁶ mm²/s, in other words, a wire withdifference greater than 300 μm (500 μm) between the maximum and minimumvalues of the wire width dimension in the results of Table 1, is furtherrolled with its width constrained.

FIG. 4 is a micrograph (magnification of 40) showing another crosssection of a wire after being rolled with its width constrained, withthe difference between the maximum and minimum values of the width ofthe wire exceeded 300 μm, in other words, the precision of the widthdimension was more than 300 μm. As shown in FIG. 4, no protrusions areformed on width-direction edges as present in FIG. 3. On the other hand,at the middle portion in the width direction of the wire shown in FIG.4, breaks are generated in some of the oxide superconducting filaments,indicating that the oxide superconducting filaments are broken. FIG. 5is a micrograph (magnification of 75) showing longitudinal cross sectionof the wire of FIG. 4. As shown in FIG. 5, it can be appreciated thatthe breaks are also present in the oxide superconducting filaments inthe longitudinal direction of the wire, that the filaments are brokenand presenting fractured layers.

FIG. 6 is a micrograph (magnification of 75) showing longitudinal crosssection of a wire after being rolled with its width constrained. Thedifference between the maximum and minimum values of the width of thewire was 300 μm or smaller, in other words, the precision of the widthdimension was 300 μm or smaller, more specifically, 50 μm. As shown inFIG. 6, no breaks are observed in the oxide superconducting filaments inthe wire, and the filaments are extending along longitudinal directionof the wire without being broken.

As described above, the wires further rolled by the four-directionalpressure rolls were thermally treated at 845° C. for 50 hours,respectively. Then, with rolling reduction of 18%, each wire was rolledfurther. Thereafter, each wire was thermally treated at 840° C. for 50hours in the atmosphere.

The critical current density at 77 K was measured for each of the oxidesuperconducting wires thus obtained. As a result, only the wire whichwas subjected to the first roll working using the lubricant havingkinematic viscosity of 20×10⁻⁶ mm²/s exhibited a critical currentdensity half that of other oxide superconducting wires, or even smaller.It can be explained that at the stage of performing first roll workingusing the lubricant having kinematic viscosity of 20×10⁻⁶ mm²/s, breakswere generated in the oxide superconducting filaments within the wire,and thus only the wire exhibited low superconducting performance. Assuch, it can be appreciated that if the difference between maximum andminimum values of width dimension of the wire as a composite exceeds 300μm before being rolled by the four-directional pressure roll with itswidth constrained, the subsequent roll working with its widthconstrained degrades superconducting performance.

FOURTH EXAMPLE

In the manufacturing method of the first example, a round shaped wirehaving a diameter of 1.5 mm was rolled to a tape-shaped wire having athickness of 0.26 mm using a lubricant having kinematic viscosity of3×10⁻⁶ mm²/s. In the roll working, the number of stages, in other wordsthe frequency of passing the composite through the pressure rolls (passfrequency), was changed to examine effects on the rolling precision.When the pass frequency was once, a round shaped wire having a diameterof 1.5 mm was rolled to a tape-shaped wire having a thickness of 0.26 mmby pressure rolls with single stage. When the pass frequency was twice,a round shaped wire having a diameter of 1.5 mm was rolled to atape-shaped wire having a thickness of 0.55 mm, and to a tape-shapedwire having a thickness of 0.26 mm, successively. When the passfrequency was four times, a round shaped wire having a diameter of 1.5mm was rolled to a tape-shaped wire having a thickness of 0.9 mm, 0.6mm, 0.4 mm, and 0.26 mm, successively. When the pass frequency was eighttimes, a round shaped wire having a diameter of 1.5 mm was rolled to atape-shaped wire having a thickness of 1.3 mm, 0.9 mm, 0.75 mm, 0.6 mm,0.5 mm, 0.4 mm, 0.3 mm, and 0.26 mm, successively.

The dimensional precision of width and thickness of the wires rolledwith each pass frequency are shown in Table 2.

TABLE 2 width (mm) thickness (mm) pass standard maximum minimum standardmaximum minimum frequency average deviation value value averagedeviation value value 1 3.68 0.03 3.81 3.64 0.259 0.004 0.265 0.250 23.70 0.03 3.80 3.65 0.260 0.004 0.265 0.252 4 3.69 0.02 3.77 3.67 0.2590.001 0.263 0.255 8 3.70 0.02 3.78 3.67 0.260 0.001 0.263 0.256

From the results of Table 2, in the similar roll working in which a 5round shaped wire having a diameter of 1.5 mm is rolled to a tape-shapedwire having an eventual thickness of 0.26 mm, variation of dimensionalprecision between the wires can also be reduced by increasing the passfrequency. Especially, it is more effective to perform roll working withthe pass frequency of more than four times. In other words, the pressurereduction of 40% or smaller in each roll working will increasedimensional precision of each wire after being rolled.

FIFTH EXAMPLE

Each roll working of the fourth example was performed using a pressureroll having grooves for widthwise constraint. Though no changes wereobserved in the variation between width or thickness of the workedwires, distributions of the oxide superconducting filaments in the crosssection of the wires were improved. As a result, the critical current Icwhich is one of the superconducting performance of the oxidesuperconducting wire exhibited improvement by 1.2 times at the liquidnitrogen temperature.

FIG. 7 shows schematic profile of the cross section along the axialdirection of a pair of pressure rolls having grooves which is used inthe manufacturing method of the present example. As shown in FIG. 7,elliptical grooves are formed at a central portion along the widthdirection of the rolls.

FIG. 8 is a micrograph (magnification of 46) showing cross section of anoxide superconducting wire obtained through roll working with flat rollsin the fourth example. FIG. 9 is a micrograph (magnification of 46)showing cross section of an oxide superconducting wire obtained throughroll working with rolls having grooves in the fifth example. As shown inFIG. 8, distribution of the oxide superconducting filaments is notuniform. In contrast thereto, as shown in FIG. 9, the oxidesuperconducting filaments are optimally distributed over the widthdirection.

SIXTH EXAMPLE

In the rolling step of the third example, roll working was performedusing mineral oil having kinematic viscosity of 3×10 mm²/s as alubricant. Resulting, changes in width against length of the obtainedoxide superconducting wire are shown in FIG. 10, changes in thicknessagainst length of the oxide superconducting wire are shown in FIG. 11,changes in roll surface temperature against length of the oxidesuperconducting wire are shown in FIG. 12. From FIGS. 10 to 12, it canbe appreciated that with a lubricant of low viscosity having kinematicviscosity of 3×10⁻⁶ mm²/s, the roll surface temperature also rises bythe processing heat generated during the roll working of the composite,and correspondingly width or thickness of the obtained wire is affected.

In comparison, a highly volatile lubricant of which boiling point isabout 200° C. and kinematic viscosity is 3×10⁻⁶ mm²/s was employed toperform roll working. Resulting changes in width against length of theobtained oxide superconducting wire are shown in FIG. 13, changes inthickness against length of the oxide superconducting wire are shown inFIG. 14, changes in roll surface temperature against length of the oxidesuperconducting wire are shown in FIG. 15. From FIGS. 13 to 15, it canbe appreciated that with a highly volatile lubricant, the increase ofthe roll surface temperature caused by the processing heat can beminimized, and correspondingly dimension of width or thickness of thewire can be kept within a prescribed range of precision.

SEVENTH EXAMPLE

To the wire manufactured by roll working with four times of passfrequency in the fourth example, further roll working was performed byfour-directional pressure rolls for widthwise constraint.

As a result, the average value of width dimension of the obtained wirewas 3.71 mm, standard deviation was 0.01 mm, maximum value was 3.73 mm,minimum value was 3.68 mm, the average value of thickness dimension was0.255 mm, standard deviation was 0.001 mm, maximum value was 0.260 mm,and minimum value was 0.253 mm. The dimension precision of width orthickness could further be improved from that of the fourth example.

INDUSTRIAL APPLICABILITY

As described above, by utilizing the manufacturing method of an oxidesuperconducting wire of the present invention, dimensional precision ofwidth or thickness of the oxide superconducting wire eventually obtainedcan be improved, and further, superconducting performance such ascritical current can be improved. Thus, when the wires produced by thepresent invention are applied to magnets or cables as practical use ofthe oxide superconducting wires, coil shape of the magnets is improved,and additionally, in case of cables, the problem of degradingsuperconducting property can be prevented without generating stress.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method of manufacturing an oxidesuperconducting wire, comprising the steps of: preparing a composite bycovering with metal powder containing an oxide superconductor or rawmaterial for an oxide superconductor; and rolling said composite using alubricant having kinematic viscosity of at most 20×10⁻⁶ mm²/s.
 2. Themethod of manufacturing an oxide superconducting wire according to claim1, wherein said kinematic viscosity is at most 10×10⁻⁶ mm²/s.
 3. Themethod of manufacturing an oxide superconducting wire according to claim1, wherein said lubricant has a boiling point of at most 200° C.
 4. Themethod of manufacturing an oxide superconducting wire according to claim1, wherein the step of rolling includes rolling said composite withpressure reduction of at most 40%.
 5. The method of manufacturing anoxide superconducting wire according to claim 1, wherein the step ofrolling includes rolling said composite using pressure rolls havinggrooves for constraining width of said composite.
 6. The method ofmanufacturing an oxide superconducting wire according to claim 1,wherein the step of rolling includes rolling said composite usingfour-directional pressure rolls.
 7. The method of manufacturing an oxidesuperconducting wire according to claim 1, wherein the step of preparingsaid composite includes filling into a metal pipe powder containing anoxide superconductor or raw material for an oxide superconductor.
 8. Themethod of manufacturing an oxide superconducting wire according to claim7, wherein the step of preparing said composite includes preparing amonofilamentary composite by filling into a first metal pipe powdercontaining an oxide superconductor or raw material for oxidesuperconductor, and preparing a multifilamentary composite by filling aplurality of said monofilamentary composites into a second metal pipe.9. The method of manufacturing an oxide superconducting wire accordingto claim 8, wherein the step of preparing said composite furtherincludes drawing said monofilamentary composite, and the step ofpreparing said multifilamentary composite includes filling a pluralityof said drawn monofilamentary composite in said second metal pipe. 10.The method of manufacturing an oxide superconducting wire according toclaim 9, wherein the step of preparing said composite further includesdrawing said multifilamentary composite.
 11. The method of manufacturingan oxide superconducting wire according to claim 1, wherein said metalpipe is a silver or silver alloy pipe.
 12. The method of manufacturingan oxide superconducting wire according to claim 1, wherein said oxidesuperconductor is a bithmuth-based oxide superconductor.
 13. The methodof manufacturing an oxide superconducting wire according to claim 12,wherein said bithmuth-based oxide superconductor includes bithmuth,lead, strontium, calcium, and copper having a (bithmuth andlead):strontium:calcium:copper composition ratio approximately expressedas 2:2:2:3.
 14. The method of manufacturing an oxide superconductingwire according to claim 1, further comprising the steps of: thermallytreating said rolled composite.